Nonlinear Fiber Optics - 4 ed. Agrawal

10.4. Parametric Amplification 397
Figure 10.15: Measured (diamonds) and calculated (solid) gain spectra as a function of signal
wavelength for a dual-pump FOPA. (After Ref. [93]; c○2003 IEE.)
zero-dispersion wavelength at 1583.5 nm with β 30 = 0.055 ps 3 /km, β 40 = 2.35 × 10 −4
ps 4 /km, and γ = 17 W −1 /km. The theoretical model included the effects of SRS to account
for the Raman-induced power transfer among the pump, signal, and idler fields.
The dual-pump configuration offers several other advantages over the single-pump
one, in addition to providing a relatively flat gain over a wide bandwidth. As the pump
wavelengths are located at the two edges of the gain spectrum, pump blocking is not
necessary when signal wavelength lies in the central flat portion of the gain spectrum.
Also, the required power for each pump to realize a certain value of gain is reduced by
50% compared with the single-pump case when both pumps are launched with equal
powers. In practice, the shorter-wavelength pump requires more power to offset the
impact of Raman-induced power transfer between the two pumps, but its power still
remains well below the level of single-pump FOPAs.
An additional advantage of dual-pump FOPAs is related to their use as wavelength
converters. Although phase modulation of the pumps is still required to suppress SBS,
spectral broadening of the idler is no longer a problem because the phases of two pumps
can be manipulated such that a specific idler used for wavelength conversion is not
broadened. For example, if the idler frequency ω 4 ≡ ω 1 + ω 2 − ω 3 is used for wavelength
conversion, the two pumps should be modulated out of phase so that the sum
φ 1 (t)+φ 2 (t) =0 at all times [94]. In the case of binary phase modulation, the two
bit streams differ in their phases by π to satisfy this condition [95]. In contrast, if the
idler at the frequency ω 7 or ω 8 is used [see Eq. (10.4.18)], the two pumps should be
modulated in phase [96].
Another advantage of the dual-pump configuration is that the state of polarization
(SOP) of the two pumps can be controlled to mitigate the dependence of the parametric
gain on the signal SOP. This problem affects all FOPAs because the FWM
process itself is strongly polarization-dependent. In the case of single-pump FOPAs, a
polarization-diversity loop is sometimes employed. In the case of dual-pump FOPAs,
the problem can be solved simply by using orthogonally polarized pumps. The polarization
effects are considered in more detail in Section 10.5.